ABSTRACT Despite the importance of maternal meiotic errors to the etiology of human chromosome abnormalities, we know very little about chromosome dynamics during human female meiosis, and remain ignorant of the reasons why the process is so error-prone. Further, we have little understanding of the basis of the sex-specific differences in meiotic error rate or recombination patterns in our species. The studies that comprise this renewal application will directly examine meiosis in human oocytes and spermatocytes. By combining this approach with molecular and quantitative methodologies, we will answer fundamental questions about meiosis in humans. We will examine and compare the way in which homologous chromosomes find and synapse with one another during meiotic prophase in the human female and male and ask how synaptic patterns influence the formation of crossovers. In addition, because the placement of crossovers established during the fetal stages of oogenesis affects chromosome segregation in the adult female, we will combine the data obtained from human studies with studies in the mouse to test different models proposed to explain the relationship between recombination and nondisjunction. Finally, we will examine the contribution of chromatin structure and interference to sex-specific differences in recombination rates. The clinical implications of the basic information obtained from these studies are considerable: Aneuploidy is the most common cause of miscarriage and birth defects in our species. Understanding the mechanics of the meiotic process in humans and sex-specific differences not only provides a crucial first step in the development of therapeutic approaches to prevent or at least minimize errors, but also critical information for the development of systems for producing gametes in vitro to treat human infertility.